Japanese Patent No. 3012249 discloses an oxidation promoting catalyst for regenerating a diesel particulate filter which traps the soot discharged from a diesel engine. The oxidation promoting catalyst is installed upstream of the diesel particulate filter of an engine exhaust passage, and promotes oxidation of nitrogen monoxide (NO) in the exhaust gas to nitrogen dioxide (NO2).
Normally, a temperature of 600° C. or more is required for combustion of soot, but in an atmosphere of nitrogen dioxide which has a powerful oxidizing ability, soot burns at a low temperature of about 275° C.
Therefore, by using an oxidation promoting catalyst, the diesel particulate filter can be regenerated even when the exhaust gas temperature is as low as 275° C.